Silica coating films are widely used as an insulation film of a semiconductor device since they have excellent insulation properties, heat resistance, abrasion resistance and corrosion resistance. Along with miniaturization of semiconductor devices, there is a demand for an insulation film material for filling narrow gaps. An insulation film used in a semiconductor device is formed by, for example, a CVD (Chemical Vapor Deposition) method or a coating method. Although a CVD method yields a good-quality film, it has a drawback in the running cost since the productivity is low and the method requires a special chemical vapor deposition device. Meanwhile, since a coating method is excellent in terms of the cost and productivity, a variety of materials have been considered in order to improve the quality of the resulting film.
There among, polysilazane are macromolecular compounds having a Si—N bond (also referred to as silazane bond) as a base unit and known as a material with which a good-quality silica-based insulation film can be formed even in a narrow gap by a relatively inexpensive method, coating method. Examples of a method of preparing an insulation film using polysilazane include those methods which comprise: (1) the coating step in which a solution of polysilazane, such as xylene or dibutyl ether solution, is coated on a semiconductor substrate or the like by spin coating or the like; (2) the drying step in which a solvent is evaporated by heating the semiconductor substrate coated with polysilazane to about 150° C.; and (3) the calcination step in which this semiconductor substrate or the like is calcinated in water vapor at about 230 to 900° C. (for example, see Patent Documents 1 and 2).
Further, a polysilazane which does not contain an organic group in the molecule (hereinafter, also referred to as “inorganic polysilazane”) is not a linear polymer constituted by a repeating base unit represented by —(SiH2—NH)—, but is a mixture of polymers having a variety of structures containing a chain part and cyclic part. It is known that the silicon atom in such an inorganic polysilazane molecule is bound with 1 to 3 hydrogen atoms, and there are also known prior arts focusing on the ratio of the SiH1 group, SiH2 group and SiH3 group in an inorganic polysilazane.
For example, in Patent Document 3, it is disclosed that a coating film having excellent heat resistance, abrasion resistance and chemical resistance as well as a high surface hardness can be obtained by using a polysilazane which has a SiH2 group ratio with respect to SiH3 group in one molecule of 2.5 to 8.4 and an element ratio of Si:N:H=50 to 70% by mass:20 to 34% by mass:5 to 9% by mass; and that the coating film can be suitably used as a binder for ceramic molded articles, particularly ceramic mold-sintered articles.
Further, Patent Document 4 discloses that a protective film having excellent mechanical strength and chemical stability is formed by coating a composition for forming a protective film of UV-shielding glass on a UV-shielding layer provided on a glass surface and heating the resultant in dry air, the composition comprising, as an indispensable component, a polysilazane which has a SiH3 group ratio of 0.13 to 0.45 with respect to the sum of SiH1 group, SiH2 group and SiH3 group in terms of the peak area ratio of 1H-NMR spectrum and a number average molecular weight of 200 to 100,000.
In addition, Patent Document 5 discloses that a coating liquid for forming an interlayer insulation film, which is composed of an inert organic solvent solution of a polysilazane adjusted to have, in terms of the peak area ratio of 1H-NMR spectrum, a SiH3 group ratio of 0.15 to 0.45 with respect to the sum of SiH1 group and SiH2 group, has excellent storage stability and coating property as well as high insulation properties; and that this coating liquid can form a fine coating film having excellent surface profile with good reproducibility. Further, it is disclosed that the adjustment can be made by substituting some of the active hydrogens of polysilazane with a trimethylsilyl group, and hexamethyldisilazane is exemplified as an adjusting agent.